1. Field of the Invention
The present invention relates to a display device comprising a scanning line driving element, a signal line driving element, and a flexible flat (Flexible Printed Circuit; FPC) cable being mounted in a frame part which is outside a display pixel region of a main substrate, and more specifically, to a mounting technique which enables to save a space of the frame part.
2. Description of the Related Art
Recently, in the field of liquid crystal display panels which are formed by sandwiching and sealing a liquid crystal layer with two glass substrates, practically used is a liquid crystal display device which comprises a scanning line driving element and a signal line driving element mounted in a region (referred to as a “frame part” hereinafter), which is out of the display pixel region in the periphery of one of the glass substrates and also is a region which is not covered by the other counter glass substrate. The driving elements are mounted to be adjacent to the longitudinal side and the lateral side of the display pixel region and on the glass substrate having the length corresponding to the lengths of the sides, a driving circuit having thin-film transistors is formed.
For example, a conventional liquid crystal display device disclosed in Japanese Patent No. 3033124 (page 5, FIG. 2) employs a structure in which a scanning line driving element having a scanning line driving circuit formed with thin-film transistors on a heat-resistant glass substrate and also a signal line driving element having a signal line driving circuit formed with thin-film transistors on a heat-resistant glass substrate are mounted in a frame part of the liquid crystal display panel.
As shown in FIG. 1 and FIG. 2, the liquid crystal display panel of the conventional liquid crystal display device is formed with a glass substrate 1 and a glass substrate 2 which are placed opposite with a liquid crystal layer in between. On the surface of the glass substrate 1, formed are a plurality of thin-film transistors for applying a voltage to display pixel electrode, a plurality of scanning lines 16 for electrically selecting the thin-film transistors, and a plurality of signal lines 17 orthogonal to the scanning lines 16. On the surface of the glass substrate 2, counter electrodes for the display pixel electrodes are formed. Further, a scanning line electrode array 16a in which a plurality of scanning line electrodes are arranged is provided in one side of the glass substrate 1, while a signal line electrode array 17a in which a plurality of signal line electrodes are arranged is provided in other side of the glass substrate 1, which crosses the side where the scanning line electrode array 16a is provided.
Further, a scanning line driving element 3 of the conventional liquid crystal display device is mounted to a frame part 16b on the scanning line electrode array 16a side of the glass substrate 1, which comprises lines of scanning line driving output terminals 18 of the driving circuit arranged by the same pitch as that of the scanning line electrode array 16a of the glass substrate 1. These output terminals 18 are connected to the respective scanning line electrodes. In the same manner, a signal line driving element 4 of the conventional liquid crystal display device is mounted to a frame part 17b on the signal line electrode array 17a side of the glass substrate 1, which comprises lines of signal line driving output terminals 20 of the driving circuit arranged by the same pitch as that of the signal line electrode array 17a of the glass substrate 1. These output terminals 20 are connected to the respective signal line electrodes.
Input terminals 19, 21 are formed in one edge part of the glass substrate of the signal line driving element, and the input terminals 19, 21 are connected to link wirings 24, 25 provided in a corner of the glass substrate 1. The link wirings 24, 25 are made of an aluminum wire and/or a chrome wire and/or a copper wire. In one edge of the corner of the glass substrate 1, connecting terminals 22, 23 being electrically connected to the link wirings 24, 25 are arranged. An FPC cable 5 for connecting to an outer circuit is connected to the connecting terminals 22, 23.
The scanning line driving output terminals 18 of the scanning line driving element 3 and the scanning line electrodes of the glass substrate 1 are arranged by the same pitch. Likewise, the signal line driving output terminals 20 of the signal line driving element 4 and the signal line electrodes of the glass substrate 1 are arranged by the same pitch. Therefore, it is possible to wire the scanning line driving output terminals 18 and the scanning line electrodes and to wire the signal line driving output terminals 20 and the signal line electrodes by the minimum distance.
However, there are some drawbacks in the liquid crystal display device disclosed in Japanese Patent No. 3033124.
A first drawback is that the width of the frame part cannot be reduced because of the width of the FPC cable for connecting to the outer circuits. The reason is as follows. In accordance with the recent technical developments, the widths of the glass substrates of the scanning line driving element and the signal line driving element have been narrowed to 4 mm or less. On the glass substrates of the scanning line driving element and the signal line driving element, it is necessary to provide wirings for supplying power supply and the like to four-system power source and GND, eighteen video signals, ten gradation voltages, sixteen control signals, two clock signals, and counter electrodes for display pixel electrodes of a liquid crystal display panel. Provided that a copper wiring (wiring width=40 μm, wiring pitch=80 μm, wiring thickness=9 μm) is provided on both faces, an FPC cable with a length of 10 cm is used for connecting to the outer circuit, and an electric current of 50 mA is flown to the four-system power sources and to the power source of the counter electrodes, the specific resistance of the copper wiring becomes 1.7×10−8 [Ω·m] for suppressing the voltage drop in the FPC cable to be 0.02 V or less. Thus, twelve wirings or more are required for the power source. Similarly, if an electric current of 100 mA is flown to the GND, twenty-four wirings or more are required for the GND so as to suppress the voltage drop in the FPC cable to be 0.02 V or less. Accordingly, there are one-hundred and thirty copper wirings or more in total for the FPC cable and sixty-five wirings or more on one face of the substrate, so that the width of the FPC cable becomes 5.2 mm or more. Thus, even though the width of the glass substrate of the driving element becomes as narrow as 4 mm or less, the width of the FPC cable is wider than this. Therefore, it is necessary to widen the width of the frame part for connecting the FPC cable.
Further, the width of the frame part cannot be narrowed since the width of the glass substrate cannot be narrowed. The reason is as follows. In accordance with the recent technical developments, it is possible to manufacture a glass substrate with the length substantially equal to the length of one side of a liquid crystal display panel. For example, in a twelve-inch XGA panel (1024×768), the length of the glass substrate of a signal line driving element is about 250 mm. Provided that two-system power source and GND are wired within the glass substrate by a copper wiring with the wiring thickness of 20 μm, and an electric current of 50 mA is flown to the two-system power source, the specific resistance of the copper wiring is 1.7×10−8 [Ω·m] for suppressing the voltage drop in the power source wiring to be 0.02 V or less. Thus, it is necessary for the copper wirings of each power source to be in the thickness of 0.53 mm or more. Accordingly, the total width of the wirings of the two-system power source and the wirings of the GND becomes 2.12 mm or more. Therefore, the width of the glass substrate cannot be reduced to less than 2.12 mm at least.
A second drawback is that there are dispersions in the transmission time of a plurality of the driving signals outputted from the driving circuit, since the length of the glass substrate is long. The reason is as follows. In accordance with the recent technical developments, it is possible to manufacture a glass substrate with the length substantially equal to one side of a display pixel region. For example, in a 12-inch XGA panel (1024×768), the length of the glass substrate of a signal line driving element is about 250 mm. Therefore, in the terminal positioned in the farthest distance from an input terminal of the glass substrate, the supplied power supply voltage drops due to the influence of the supply wiring resistance. Thus, the delay time of the outputted driving signals is increased.
A third drawback is that it becomes difficult to align the positions of the terminals with each other when mounting the scanning line driving element and the signal line driving element onto the frame part, because of the structure in which the input terminals are positioned in the shorter side of the glass substrates of each driving element and the output terminals are positioned in the longer side. The reason is as follows. In accordance with the recent technical developments, the width of the glass substrate is narrowed to be as small as 4 mm or less. However, there are forty input signals or more in the signal line driving element so that the pitch of the input terminals becomes 100 μm or less. Further, in accordance with the improved resolution of the liquid crystal display panel, the pitch of the signal lines in the liquid crystal display panel is also narrowed. For example, in a 12-inch XGA panel (1024×768), the pitch of the output terminals in the signal line driving element becomes 80 μm.
A fourth drawback is that the reliability of the display device becomes insufficient, which depends on the reliabilities of the scanning line driving element and the signal line driving element. The reason is as follows. In accordance with the recent technical developments, thin-film transistors formed on the glass substrate are micronized so that the number of the thin-film transistors which can be formed on a single glass substrate is increased. For example, the number of the thin-film transistors mounted to the signal line driving element is 2,000,000 or more. Therefore, the rate of generating defected glass substrate due to the defect of the thin-film transistors is increased.
A fifth drawback is that the drop in the power supply voltage within the glass substrate of the driving element is increased due to an increase in the scale of the circuit of the glass substrate. The reason is as follows. In accordance with the recent technical developments, the thin-film transistors forming the driving circuit glass substrate are micronized to be as fine as some μm or less so that the number of the thin-film transistors formed on a single glass substrate is increased. For example, the number of the thin-film transistors forming the glass substrate of the signal line driving circuit is 2,000,000 or more. Therefore, the current consumption within the glass substrate is increased and the power supply voltage to be supplied is dropped due to the influence of the supply wiring resistance.